The electric power steering system, mounted in an automotive vehicle for example, includes an electric motor connected to a steering mechanism extending from an steering member to dirigible wheels, and is adapted to apply a motor power to the steering mechanism thereby assisting the driver in the steering operation by way of the steering member.
The electric motor may encounter a problem that an output torque thereof suffers ripple (pulsation). The ripple is classified broadly into cogging toque (mechanical ripple) resulting from motor constitution including the number of rotor poles, the number of slots for stator coils and the like; and electrical ripple occurring as a result of induced electromotive force having waveform distorted relative to an idealistic waveform. Such torque ripples contained in the motor output constitute one causative factor of degrading a steering feeling characteristic of the steering system. This leads to a strong demand for suppressing the torque ripples in the steering system.
As disclosed in Japanese Unexamined Patent Publication No. 2003-61272, for example, some conventional systems are adapted to reduce the torque ripple by changing the configuration of the slot at its portion opposite a rotor magnet or adjusting a skew angle.
By the way, the electric power steering system employs, for example, a three-phase brushless motor as the above electric motor.
However, the aforesaid motor may sometimes provide an output torque containing the torque ripple (electrical ripple) caused by high-order components of current produced by the rotation of a distorted magnetic field, thus degrading the steering feeling.
In view of the aforementioned problem encountered by the conventional technique, it is an object of the invention to provide an electric power steering system capable of suppressing the torque ripple caused by the high-order components of current, thereby preventing the degradation of steering feeling.